This invention relates to an optical transmission system in which communication signals are sent over a common optical fiber path at different wavelengths, or colours. Such a system is termed wavelength division multiplex (WDM), and it permits efficient use of an optical fiber path.
Fiber group velocity dispersion was the most significant problem when single channel systems, operating near the minimum of fiber attenuation (about 1550 nm), were introduced some years ago. To overcome this problem, dispersion-shifted fibres were designed, having the minimum of group velocity dispersion occurring near 1550 nm. Fiber of this kind has been widely installed for optical transport networks. The introduction of the optical amplifier and progresses in lasers manufacturing now makes it possible to carry many high bit-rate channels on very long distances at different wavelengths (WDM). Unfortunately non-linear fiber behaviour is significant at high powers boosted into the fiber by optical amplifiers and has a big impact on bit error rates (BER) performances, giving rise to cross talk among channels.
It has proved difficult substantially to raise the power of optical signals which are launched into a WDM system without causing an unacceptable bit error rate.
The present invention seeks to provide a method of operating an optical transmission system in a more efficient manner.
According to a first aspect of this invention, a method of determining the launch power of a WDM optical transmission system having a plurality of sequential amplified spans which together form a link includes the step of setting the amplifier power at each amplifier at or near the same maximum value at which both the optical signal to noise ratio due to amplified spontaneous emission (OSNRase) and the optical signal to noise ratio due to four wave mixing (OSNRfwm) each are above respective predetermined threshold values at the end of the link.
For a given launch power at the beginning of each span of the link the actual values of OSNRase and OSNRfwm are inter-related, and preferably an iterative process is used to determine the optimum maximum launch power which does not give rise to an unacceptably high bit error rate.
According to a second aspect of this invention, a method of determining the launch power of a WDM optical transmission system having an amplified link includes the steps of
(a) setting the amplifier power at the beginning of the link,
(b) determining the signal to noise ratio due to amplified spontaneous emission (OSNRase) and the optical signal to noise ratio due to four wave mixing (OSNRfwm) at the end of the link,
(c) comparing each of OSNRase and OSNRfwm with a respective predetermined threshold value,
(d) altering the amplifier power,
(e) repeating steps (b), (c) and (d) until the amplifier power is at or near a maximum value whilst both OSNRase and OSNRfwm exceed the respective threshold values.
Preferably the link includes a plurality of sequential amplified spans, in which case preferably each amplifier power is at the same value.